There is always a demand for increased accuracy in the measurement of electrical parameters, such as voltage and current or phase and power. In order to ensure and maintain accuracy of such measurements, an electrical measuring device or unit under test (UUT), such a voltage meter, current meter, power meter, etc, will itself have to be calibrated with reference to a standard device whose calibration can be traced back to national standards.
An example of this is the need to measure electrical power in mains electricity generation and supply. In recent years, following deregulation of the electrical generation and distribution networks, there has been an increased demand for the accurate measurement and monitoring of electrical power, both on domestic and industrial scales of power consumption. There is also the increased need for increased accuracy of measurement in electricity traded between electricity generators. However, at the same time, main electrical power has become “dirtier”, with increased high frequency noise and low frequency flicker, making measurement more difficult, particularly quantifying variability in the electrical supply and errors in the measurement of electrical power.
An electric power meter or revenue meter (kWh meter) may be calibrated in various ways. Two common ways of calibrating an electric power meter involve either placing the standard meter in a series and/or parallel connection with the UUT and then making simultaneous measurements, or making sequential measurements by switching or substitution of the standard meter and the UUT. The first is to substitute a more accurate voltage and/or current measuring device in place of the meter being calibrated. In order to calculate the power, it is also necessary to know or to measure the phase angle between the voltage and current being measured. Once the measurement has been performed this is then compared with the measurement provided by the meter. The meter is then adjusted to agree with the measurement made by the standard device. A problem with this method is that the electrical output of the source may change between measurements.
An alternative is therefore to use both the standard device and the meter at the same time to make the measurements. This, however, creates problems owing to the possible interaction between the standard device and meter.
There are a number of problems with this approach, particularly when the measurements are being performed on the mains power supply or the output of an electrical generator. Partly, this is because the calibration is done under the particular conditions prevailing at the time of the calibration. It may be desired to calibrate the meter under a wider range of conditions. For example, harmonic distortion in the mains power may vary depending on the time of day, and it may therefore be desirable to perform measurements under all possible conditions, including conditions in which various types of harmonic distortion are present.
Problems may also arise if the standard meter and the UUT do not present equivalent loads to the electrical source.
One technique used to measure low frequency flicker is to simulate this by switching a load to develop a potential difference across a reference impedance. Careful selection of the reference impedance and the switched impedances yield signals of sufficient accuracy to calibrate flicker meters. Such measurements can also be adversely affected if there is noise on the mains power, or if the source impedance is not zero, or if there is harmonic distortion of the mains supply.
There is a growing requirement to characterize and verify the performance of power measurement instruments in the presence of differing forms of distortion and anomalies. The standards IEC61000-4-30 (currently in draft form) and IEEE P1159.1 both require that power quality measurement instruments are calibrated in the presence of simultaneous, multiple forms of anomalous signal. For example, a flicker measurement is calibrated in the presence of controlled amounts of harmonic distortion. While the limitations in accuracy described above may be acceptable in some cases, none of the approaches outlined above is able to provide the facility for calibration when simultaneous degradations in the mains signal are present.
It has therefore been proposed to use a known source to calibrate an electrical measurement device. In the case of calibrating a power meter, it is therefore necessary to generate an electrical test signal having known electrical characteristics, which can be traced back to national standards. The accuracy of the calibration will therefore depend on the accuracy of the signal generation, and the usefulness of the source will depend on the ability to generate a wide range of signal characteristics within the required calibration accuracy. Prior art signal generation devices have not been sufficiently accurate and/or quick to use. Because of the difficulty generating suitable signals, standard signal sources have not been widely used in applications such as the measurement of mains power characteristics.